The present invention relates to a catalyst for purifying exhaust gas.
A catalyst for purifying exhaust gas comprising a combination of a zeolite having platinum deposited thereon and CeO2 is proposed in JP-A-8-131838 as a catalyst improved in the ability to purify NOx (nitrogen oxides) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). The platinum in the above catalyst functions as a catalyst metal and has the ability to oxidize and reduce the exhaust gas. The oxidizing ability of the platinum contributes to oxidation reactions such as HC (hydrocarbon)+O2xe2x86x92H2O+CO2 and CO+O2xe2x86x92CO2. At a theoretical air/fuel ratio, the platinum adsorbs NO and the reducing ability thereof contributes to reduction reactions such as NOxe2x86x92N2. In oxygen-excess atmospheres, the platinum contributes to oxidation reactions such as NO+O2xe2x86x92NO2 and reduction reactions such as NO2+HC+O2xe2x86x92N2+CO2+H2O.
The zeolite functions to adsorb HC in the exhaust gas and feed the concentrated HC to the platinum. Thus, the percentage of NOx purification in oxygen-excess atmospheres can be heightened. CeO2 has the ability to adsorb NOx in oxygen-excess atmospheres to thereby heighten the concentration of NOx around the platinum. This also can bring about an improvement in the percentage of Nox purification in oxygen-excess atmospheres. CeO2 further has the effect of inhibiting the thermal deterioration of the catalyst.
The present inventors made various investigations on the catalyst described above. As a result, they have concluded that the above-described catalyst, upon exposure to a high-temperature environment containing oxygen and steam, i.e., a hydrothermal environment, deteriorates in the ability to purify NOx in oxygen-excess atmospheres, and this is attributable to the poor durability of the zeolite in the hydrothermal environment.
The catalyst described above shows the relatively satisfactory ability to purify NOx when the temperature of the exhaust gas, i.e., gas temperature, is low, specifically, in the range of from 150 to 300xc2x0 C. However, in a high-gas-temperature range above 300xc2x0 C., the catalyst shows the property of releasing NOx.
The reasons for the above phenomenon are thought to be as follows. The purification of NOx in the low-temperature range is based on the mechanism of adsorption of NOx and reduction of the NOx. Consequently, when the amount of adsorbed NOx is larger than the amount of reduced NOx, the NOx remaining unreduced is occluded by the catalyst. This occluded NOx is released in that high-temperature range.
As a result of the release of occluded NOx in the high-temperature range, the exhaust gas comes to contain the released NOx besides the NOx originally generated by engine running. In addition, the amount of the NOx thus released is not constant. Because of these, there has been a problem that the control of NOx purification is difficult.
An object of the present invention is to provide a catalyst for purifying exhaust gas which can retain the high ability to purify NOx in oxygen-excess atmospheres even after exposure to a hydrothermal environment such as that described above and does not release NOx in the high-temperature range.
In order to accomplish the above object, the present invention provides a catalyst for purifying exhaust gas which comprises a combination of a carrier having platinum deposited thereon and CeO2 having an average crystallite diameter (D) satisfying Dxe2x89xa713.0 nm, the carrier being made of a gallosilicate having an SiO2/Ga2O3 molar ratio (M) satisfying 100xe2x89xa6Mxe2x89xa61,230.